Transition slot exposure
Transition slot exposure is critical because it affects both idle quality, idle air / fuel mixture as well as 'tip-in' or 'off-idle' response.
Too little or no transition slot exposure on the primary side on most engines will cause a stumble or hesitation on tip-in. Viewed with the butterflies in the idle position, base inverted.
Too much transition slot exposure on the primary side on most engines will cause: a rich idle / exhaust smell; little to no sensitivity to idle mixture screw adjustment; soft tip-in stumble or hesitation on tip-in or part throttle response.
The following picture shows a near perfect transition slot exposure for most engines. The shape of the transition slot exposed (under the butterfly) will look something like a square.
To adjust secondary transition slot exposure a secondary idle screw / throttle stop is provided. On TMP Carbs' preppared carbs, the idle speed screw is accessible from the top. To access it, with engine off, simply open the throttle to expose the secondary idle speed screw and turn.
Click on picture to enlarge
Replacement of "idle fuel restrictions" (IFRs) also referred to as "idle jets".
IFR's (idle jets) on TMP Carbs' prepared Holley carburetors utilizing Holley metering blocks are located as indicated in the diagonal passage intersecting the idle well providing communication to the idle passage in the metering block face. TMP uses 6x32 brass set screws for the IFRs.
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Here is a another view showing the angle of incident and location of the IFRs.
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Power Valve Tuning
Power valve tuning is critical for best mileage, throttle response and performance. By far, most high performance and racing engines require a late opening, low value power valve such as a 2.5 or 3.5. The proper power valve value can be determined by measuring manifold vacuum when accelerating from a dead stop or low speed taking note of the vacuum reading as the secondary barrels begin to open.
